LEDs have been widely used for decorative lighting purposes because of their low cost, low electrical power consumption, and long life time. For example, LED light strings have taken the place of many conventional incandescent light strings.
Different from conventional incandescent bulbs that are not sensitive to the electrical current direction, LEDs work only with current in a specific (positive) direction. Therefore, if an LED light string is connected to the household AC power supply directly, the LED bulbs emit light only during the positive half waves and the LEDs produce a 60 Hz glittering. A standard solution to this problem is to insert an AC-DC converter between the AC power supply and the LED string. As implemented by most of existing LED strings, such an AC-DC converter typically takes a four-diode full-wave rectifying circuit.